Should I buy Sony Vegas Movie Studio HD 11 or 12...

All versions of Vegas (and any full-function video editor) have problems under certain circumstances. If you let that scare you off then you'll never edit any video.

I've been using Movie Studio since V8 and I still think it's the best editor out there for real HD (1920x1080.) V12 has just had it's first update, so that's the one to get. Be sure you download the 64-bit version since it is different from the 32-bit one.

PS: for some totally obscure marketing reason Sony has stopped using the Vegas term for referring to this product. Now it is simply Movie Studio. There are lots of questions about why Sony did this; so far I've not seen any believable explanation.